Lubricant compositions used as a motor oil conventionally comprise from 50 to 90% of base oils, which may be of mineral, synthetic or natural origin, and additives. These additives are typically detergents, dispersants, antioxidants, polymers enhancing the viscosity index (VI) (VI improver), friction modifiers, flowpoint-lowering agents, anti-foam agents . . . . The synthetic bases are for example polyalphaolefins, gas-to-liquids (GTL) bases for example obtained by Fischer-Tropsch methods, or certain esters.
Another category of synthetic base oils is formed by polyalkylene glycols (PAGs). These latter bases are for example obtained by polymerization or copolymerization of alkylene oxides, preferentially comprising between 2 and 8 carbon atoms. Methods for producing PAG bases are for example described in applications WO2009/134716 and WO2009/134638.
The use of PAG bases in motor oils, notably for automobile engines, has a certain number of benefits, notably because of their good tribological and rheological properties, but also because they are polar bases. They are therefore capable of properly solubilizing certain additives required for formulating a motor oil on the one hand, which would make these additives efficient at lower contents, whence an economic gain. On the other hand, they are capable of also solubilizing solid contaminants which are formed during the use of the oil in the engine, whence a gain in engine cleanliness.
However, the use of these PAG bases in an engine lubricant comes up against a difficulty. Their Noack volatility, measured according to the CEG L 40-93 standard is very high which does not allow their incorporation in motor oil formulations, notably in a large amount and even less as exclusive base oils. Another difficulty comes from the low solubility of PAG bases in oily compounds, notably in the other base oils, which does not make them very suitable for use in a motor oil formulation. The PAGs are mainly known for their use as base fluids in oils for compressors in cooling systems. However, the state of the art contains a few examples of other uses of PAGs, for example in motor oils, for example automobile or marine engines.
Application WO2009/134716 thus discloses an oil for an automobile engine comprising at least one polyalkylene glycol base oil adapted to a use in automobile engines, combined with an additive package comprising derivatives of aspartic acid, and optionally anti-wear additives, anti-corrosion agents, antioxidants, friction modifiers, anti-foam agents. No mention is made of the presence of detergent additives in said compositions.
Application WO2009/134638 discloses a lubricant composition for an internal combustion engine comprising polyalkylene glycols obtained by copolymerization of ethylene oxide and of an alkylene oxide having up to 12 carbon atoms, comprising between 1 and 50 alkylene oxide units, and optionally a VI enhancer polymer. No mention is made of other additives present in the disclosed compositions.
Application FR 2817874 discloses functional fluids for automobiles, notably for automobile engines, comprising polyalkylene glycol bases and a reducing agent which may be urea. These PAGs are in particular obtained from ethylene and propylene oxide. No specific mention is made of other PAG bases, nor of other additives used in combination with the PAG bases.
U.S. Pat. No. 5,885,555 discloses a composition for the formulation of toothpastes comprising polyalkylene glycols and methyl salicylate, a so-called “wintergreen” toothpastes. The latter compound is not at all similar to the detergents of the salicylate type used in lubricant compositions, for example in motor oils, where the benzene ring of salicylic acid comprises hydrocarbon substituants with sufficiently long chains for forming the lypophilic portion of the detergent.
Application EP 1 990 400 discloses lubricant compositions for transmissions in marine applications, comprising a base oil which may be a synthetic or natural oil. The synthetic oils may be polyalphaolefins (PAO), various esters of (di)carboxylic acids and alcohols or polyols, alkyl benzene bases, polysilicons, polymer or copolymer olefins, polyphenols, alkyl diphenyl ethers, alkyl diphenyl sulfides, polyalkylene glycols, Fischer Tropsch bases. These compositions may also comprise at least one metal detergent, which may be selected from sulfonates, phenates, sulfurized phenates and salicylates. No specific combination of PAG bases with salicylates is disclosed. These compositions for marine transmissions exhibit good water emulsification properties as well as anti-wear properties.
Application JP 2007 204451 discloses a polyalkylene glycol which is a tetraether obtained by polymerization of propylene oxide, as well as a diether, for use as a base oil in lubricant compositions, notably for motor oils and in the presence of certain additives, notably detergents. The disclosed tetraethers however have quite low molecular weight and low kinematic viscosity at 100° C. Their use in formulations of motor oil therefore requires their use in combination with other base oils; now these tetraethers only include propylene oxide units, which do not give them much solubility in oils. Moreover, solubilization or keeping them suspended in tetraether bases of additives including a lipophilic portion, such as for example detergents, is likely to pose a problem.
The diether as for it consists of 3 long (C6-C14) carbon chains, delimited by two ether functions. The two oxygen atoms of the ether functions are separated from each other by 5 carbon atoms. These bases are therefore not polyalkylene glycols, where by nature, the oxygen atoms are separated by 2 carbon atoms, which corresponds to the attack of the epoxy function of the alkylene oxides during polymerization. It is foreseen that their behavior, notably in terms of polarity, will not have the same advantages as PAGs in an engine formulation. Therefore, there exists a need for lubricant compositions comprising a significant portion of PAG bases, which may comprise so-called oily or lipophilic compounds which are miscible with said PAG bases, and the Noack volatility of which is compatible with a use as motor oil.
For engine lubricants, the Noack volatility, measured according to the CEC L 40-93 standard, is typically less than 15%, preferentially less than 13%. It is typically comprised between 8 and 15%, or further between 10 and 13%. Surprisingly, the applicant noticed that the Noack volatility of certain PAG bases, moreover suitable for use in motor oil, is strongly reduced when they are combined with specific additives. Without having the intention of being bound to any theory, it seems that these additives play a role either on inhibiting the formation of volatile degradation products of the PAGs when these bases are subject to conditions for analyzing Noack volatility, or on stabilizing these degradation products in the oil matrix.
The present invention therefore relates to lubricant compositions comprising these specific combinations of PAG bases and of additives, as well as to their use as a motor oil, notably for automobile vehicles. The present invention also relates to the use of these additives or of additives package containing them in motor oil formulations comprising PAG bases. Finally the present invention also relates to the use of these additives, or of additives packages containing them, in order to reduce the Noack volatility measured according to the CEC L 40-93 standard, of PAG bases.